The present invention relates to the field of integrated circuit memories and, in particular, to alpha radiation soft-error immune memories.
Alpha radiation can cause "soft" or random errors in integrated circuit memories An alpha "hit" generates an instantaneous current pulse in a memory cell that can cause the cell to change state. A model of current generated by an alpha hit is described in "Current Modeling of Alpha-particle Induced Soft Errors in Bipolar Memories" by Zhang and McCall (Proceedings of the 1987 Bipolar Circuits and Technology Meeting, pp 168). This problem exists across various integrated circuit technologies including bipolar and field effect transistor (FET) technologies.
An alpha-induced current of even very short duration can be sufficient to switch the state of a memory cell. To prevent loss of state due to an alpha event, the most common approaches have been to add a large storage node capacitance, to add a PNP load element, or to protect the memory with a die coat. In addition to requiring more complexity in the fabrication process, such techniques to improve alpha immunity often require trading off the density and speed-power performance of the memory cell.
In addition to alpha sensitivity, conventional bipolar memory circuits also suffer from write recovery delay, i.e. the recovery time necessary after a write operation before a read operation can be performed. That delay reduces the overall performance of a memory system. No circuit methods have been disclosed to date that address both the alpha-sensitivity and write recovery delay. Accordingly, a need remains for a better integrated memory cell.